Four hundred years ago, the famous astronomer Johannes Kepler (best known as the discoverer of the laws of planetary motion), was startled by the sudden appearance of a "nova" of "new star" in the western sky, rivaling the brilliance of the nearby planets.

Although the nova was new to the night sky, the event was actually the death of an old star, now called a supernova.

Kepler's supernova is the most recent supernova observed in our Milky Way galaxy. There are actually two ways to make a supernova. One type is like the Crab Nebula, the explosive death of a giant star when in runs out of nuclear fuel. The second type involves a star called a white dwarf. A white dwarf is the remains of a small star, like the Sun, when that star runs out of nuclear fuel. The Sun will become a white dwarf roughly five billion years from now. If the white dwarf is alone in space, that's where its story ends. But if the white dwarf has a companion star, then its gravity can strip away the outer layers of the companion, cloaking itself in a thick layer of hydrogen gas. The layer of hydrogen around the white dwarf gets thicker and hotter, creating a ticking hydrogen bomb. When a critical temperature is reached, the white dwarf blows itself to pieces in a supernova explosion. Evidence suggests that Kepler's Supernova of 1604 was the white dwarf type.

Figure 18: Kepler's Supernova - visible image. This image, taken by the Hubble Space Telescope, reveals places where the supernova shock wave is slamming into the densest regions of surrounding gas. Touch the glowing knots of gas represented as raised irregular shapes, although there isn't enough detail at visible wavelengths to infer a shape or structure.

Figure 19: Kepler's Supernova - infrared image. The Spitzer Space Telescope is sensitive enough to detect both the densest regions of the Kepler supernova remnant, as seen by Hubble, plus the entire expanding shock wave. The infrared image shows a dot pattern for the glow from heated microscopic dust particles that have been swept up by the supernova shock wave. As you touch the infrared image you can sense more structure, especially an arc to the northeast.

Figure 20: Kepler's Supernova - X-ray image. The Chandra X-ray image depicts regions of very hot gas radiating higher-energy X-rays directly behind the shock front. As you explore the outer boundary of the pattern, you identify a more complete ring structure. This is the expanding shell of material from the supernova explosion defined with a long dash horizontal texture. The X-ray emission comes from the gas as it collides with the surrounding interstellar material; this is unlike the Crab Nebula, where a pulsar powers the X-ray emission. A supernova of the white dwarf type does not leave a remnant behind.

Figure 21: Kepler's Supernova - three different textures represent this combined visible, infrared and X-ray image. When we combine the three previous images and add in an image of low-energy X-rays, we get a rich and detailed image of the structure of Kepler's Supernova. This combined image unveils a bubble-shaped shroud of gas and dust, 14 light-years wide and expanding at six million kilometers per hour (about 3.6 million miles per hour). This image is a great example of the power of multi-wavelength observations.

The objects we have studied so far have been stars and former stars in our own Milky Way galaxy. The Milky Way is made up of around 300 billion stars, and a supernova occurs in the Milky Way every thirty years or so. But the Milky Way is just one galaxy, a city of stars in a universe filled with galaxies. Let's now move up in scale, and voyage through the realm of the galaxies.